The present invention relates to plastic optical fibres and to their production process. These fibres are used for optical transmission purposes, particularly in medicine, telecommunications, the automobile industry, oceanography, display means, for decorative purposes, particle physics, etc.
These optical fibres are generally formed from a first material constituting the fibre core and a second material surrounding the first and constituting the fibre sheath. The core material in which the useful light energy propagates must have good light transmission characteristics and has a refractive index higher than that of the sheath material.
In such optical fibres, light propagates by total reflection in the core--sheath interface. This type of propagation makes it possible to transmit light quantities which increase in proportion with the difference between the refractive indices of the core material and the sheath.
The core material is generally constituted by glass or silica, these materials having good optical transmission characteristics. However, these fibres have the disadvantage of being expensive, heavy and relatively inflexible. Moreover, during the last few years, various attempts have been made to produce optical fibres from plastics material.
The hitherto known plastic optical fibres are constituted by transparent materials of the amorphous type, such as e.g. polymethyl methacrylate, known as PMMA, which is generally used as a core material. Thus, PMMA has a high transparency and a good flexibility. However, it has a relatively low refractive index (1.48-1.50), which makes it necessary to use a sheath material with a lower refractive index. However, low index polymers are extremely rare. The polymers which can be used are fluorinated polymers, such as 2,2,2-trifluoroethyl polymethacrylate, which has a refractive index close to 1.41.
Optical fibres constituted by this material as the sheath material and PMMA as the core material, together with their production process are described in the 1981 Japanese Patent No. 56 8321.
Other fluorinated compounds usable as the sheath material, with PMMA as the core material are described in French Patent No. 2,493,997.
Unfortunately, the production process of these fibres by coextrusion using a double die suffers from shortcomings, such as wear to the die and the attachment of impurities, which leads to a deterioration of the total reflection surface and the risk of a mediocre fibre quality in industrial production.
Another presently used core material is polystyrene, which has the advantage of a high refractive index (1.58-1.62). An example of optical fibres using such a sheath material is illustrated in the article in J. Appl. Phys, 52(2) of December 1981, pp.7061-7063, entitled "Low-loss polystyrenecore-optical fibres". In this example, the sheath material used is a vinyl acetate--ethylene copolymer. However, these optical fibres unfortunately have mediocre mechanical properties.